The present invention relates to a method of decomposing organochlorine compounds such as dioxins reductively or catalytically in an autoclave.
Organochlorine compounds such as dioxins and polychlorinated biphenyls pollute air, river water, groundwater, soil and the like. In Aparticular, organochlorine compounds having toxicity such as carcinogenicity have problems from the viewpoint of environmental pollution. A technique for suppressing discharge of these organochlorine compounds and a technique for decomposing organochlorine compounds existing in the environment in the form of pollutants after discharge have been developed.
In general, organochlorine compounds which cause environmental problems are difficult to decompose naturally. Known methods of making the organochlorine compounds harmless are as follows; a) a method of decomposition with ultraviolet radiation, electron radiation or radial rays, b) a method of decomposition with microorganism, c) a method of decomposition by combustion, d) a method of chemical decomposition with an oxidizing agent, e) a method of oxidative decomposition with supercritical water and the like.
However, the method of decomposition with ultraviolet radiation, electron radiation or radial rays has a disadvantage in that a cost is high or decomposition efficiency is low. In the method of decomposition with the microorganism, decomposition efficiency and a decomposition rate are low. In the method of decomposition by combustion, highly poisonous substances such as dioxins are likely to be generated reversely depending on a combustion condition. In the method of chemical decomposition with the oxidizing agent, the organochlorine compounds can be decomposed in several hours, but corrosion of apparatus materials with the oxidizing agent leads to problems. The method of oxidative decomposition with supercritical water needs too high energy.
An object of the present invention is to provide a method of decomposing the organochlorine compounds such as dioxins which can solve the above-mentioned various problems of the prior arts by decomposing the organochlorine compounds reductively or catalytically.
A method of decomposing organochlorine compounds according to the present invention is a method characterized in that organochlorine compounds such as dioxins and o-chloroanisole are decomposed in an aqueous alkali solution in the presence of a reducing agent and/or a catalyst.
The alkalis which can be used in the present invention are hydroxides and carbonates of alkali metals or alkaline earth metals and the like, and preferably at least one selected from the group consisting of sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate and aqueous ammonia in terms of working environment.
A preferred reducing agent which can be used in the present invention is at least one selected from the group consisting of sodium hydrosulfite, ascorbic acid, hydrazine, hydrazine hydrate, neutral hydrazine sulfate, hydrazine carbonate, sodium thiosulfate, sodium sulfite, potassium sulfite, hydroquinone, 4-methylaminophenol sulfate and Rongalite. They are made harmless during the decomposition treatment.
A preferred catalyst which can be used in the present invention is at least one selected from the group consisting of activated carbon and titanium oxide.
When the organochlorine compound is water-insoluble, it is preferable to add a water-soluble organic solvent to the aqueous solution and thereby dissolving the organochlorine compound in water. Examples of the water-soluble organic solvent can be acetone, methanol and ethanol.
It is preferable to use the reducing agent and/or the catalyst in excess, for example, in an amount (mole) of 1 to 2.5 times the amount of the organochlorine compound.
Decomposition-treatment temperature is preferably 200xc2x0 to 400xc2x0 C.
It is preferable to neutralize excess alkali after the decomposition-treatment with a mineral acid such as hydrochloric acid or sulfuric acid.
It is preferable to treat an excess reducing agent after the decomposition-treatment with air, oxygen or an oxidizing agent such as aqueous ozone or aqueous hydrogen peroxide.
It is preferable to use an autoclave as a decomposition tank.
Since the organochlorine compounds such as dioxins are decomposed reductively in the aqueous alkali solution in the method of the present invention, generated chlorine and hydrogen chloride are absorbed by the alkali so that the method does not cause corrosion problems of apparatus materials and the like.
When the reducing agent is used, one has only to oxidize the excess reducing agent and neutralize the excess alkali after the treatment. Accordingly, a treatment cost can be suppressed.